A variety of different technologies have been developed to produce high-resolution video images. For example, a standard spatial light modulator (SLM) has been developed that uses a two-dimensional array of mirrors. Each mirror can be modulated on and off to produce a pixel of light in the final image. In order to produce an image having a resolution sufficient for high-definition television (HDTV), the SLM can have over two-million independently moving mirrors. Producing an SLM chip that has the requisite number of mirrors can be costly. Moreover, developing a full color SLM using three chips, one for each primary color, has been prohibitively expensive for the consumer market. Projectors using three SLM chips typically cost tens of thousands of dollars.
Column modulators were developed, in part, to overcome the limitations of the SLM. Column modulators are able to produce a 1×N column display of pixels at once rather than using independent mirrors for each pixel. The 1×N column of pixels is referred to as a line image. A plurality of line images can be combined to create a video image. For example, an HDTV display can comprise an image that is 1080 pixels high by 1920 pixels wide. Displays that use column modulators, such as the Grating Light Valve (GLV), as disclosed in U.S. Pat. No. 5,311,360, function by scanning a continuously modulated column (of 1080 pixels in the case of HDTV) across a viewing surface in a time that is considerably shorter than the persistence of vision, so that an apparent two-dimensional image (1080 pixels by 1920 pixels) is created from the scanned line image. Because only 1080 pixels need to be controlled, in this example, rather than over two million, as in the SLM example above, the column modulator can be simpler and less expensive to manufacture and control.
The more-efficient design of the column modulator makes it an optimal choice when a very high-resolution video display is needed, such as a large public video display or a large simulator where near-photo-realistic video is desired. Column modulators can be used to produce video displays having substantially higher resolution than HDTV standards. However, increasing the resolution of a video display also decreases the tolerance for producing a display that is in focus. Depth of focus is the distance the display surface can move and still be in focus. The depth of focus is dependent on the pixel size. As the pixels become smaller in a high-resolution display, the depth of focus tolerance becomes tighter. A high-resolution display can have a depth of focus with a tolerance of less than ±10 mm.
There are various schemes discussed in the patent literature for scanning a substantially one-dimensional spot image (i.e., rastering). But there are no descriptions to date concerning the specification and design of optical systems for distortion-free scanning of two-dimensional line images on a surface having a predetermined shape. The differences in requirements between rastering and line scanning are great enough that the approaches covered in the patent literature for rastering a spot image are generally not applicable to the problem of scanning a high-resolution column image to produce two-dimensional images.